Technical > Matt Shaff's Engine Shop

Compression ratio question

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Supercat:
What is a good compression ratio for a blown 477 cu.in. alcohol engine?

GlennLever:

--- Quote from: Supercat on January 21, 2018, 10:52:46 AM ---What is a good compression ratio for a blown 477 cu.in. alcohol engine?

--- End quote ---

That is a hard question and I do not profess to know the answer.

What I have heard is there are really two schools of thought.

1) keep the compression relatively low in the engine (9:1) and let the blower do all the work.
2) Let the compression be higher in the engine 11.5:1) and do not push the blower as hard.

My last engine I ran for 5 seasons with an engine that was set up al wrong for a blower (theoretically (11.5:1, long stroke, smallish piston bore) .

My new engine has less compression, shorter stroke and bigger bore.

 

Time will tell.

Supercat:
The new 477 is a 4.500 bore and a short 3.750 stroke.
The heads flow 500+cfm

Paul New:
It all depends on your tuning window. A high compression blown engine can be finicky, go from safe tune to detonation. I have been anywhere from 10-1 all the way to 11.8-1 on my SBC’s

wideopen231:
Lots of thing add to correct answer. Combustion chamber design,amount of boost are few of them. Our Hemi stuff ran 12 to 13:1. Agree that going on lower side helps tunning window.Plus with blower you can always up drive for more power. IMO 11 or 12 :1 works good. If small chamber heads I would probably go little less and tune to it.

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